1. Field of the Invention
The present invention relates generally to a proportioning valve assembly which is suitable for use in an actively controlled automotive suspension system. More specifically, the invention relates to an electric circuit associated with the proportioning valve for controlling valve position and whereby controlling pressure supplied to a work therefrom. The invention also relates to an actively controlled suspension system which employs the proportioning valve assembly as a pressure control valve for adjusting suspension characteristics to regulate vehicular height and vehicular attitude.
2. Description of the Background Art
U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987, the co-pending U.S. patent applications Ser. Nos. 052,934, 059,888, 061,368, 060,856 and 060,909 respectively filed on May 22, 1987, Jun. 9, 1987, Jun. 15, 1987, Jun. 12, 1987 and Jun. 12, 1987, and all assigned to the common assignee to the present invention disclose actively controlled suspension systems which employ pressure control valves for adjusting pressure in a working chamber of a hydraulic cylinder. The pressure control valve employed in such an actively controlled suspension system defines a pilot chamber and a feedback chamber to which output fluid pressure is fed back from an outlet so as to adjust the fluid pressure to be discharged from the outlet toward the pilot pressure. The pilot pressure is adjusted by an electronic actuator, such as a proportioning solenoid. In the actively controlled suspension system, attitude change suppressive suspension control is performed by adjusting the pilot pressure and whereby adjusting the suspension characteristics between harder suspension characteristics and softer suspension characteristics. Such adjustment is performed by adjusting a position of the valve member associated with the pilot chamber for adjusting the pilot pressure.
On the other hand, the actively controlled suspension system also performed absorption of road shock by absorbing vibration energy input through the vehicular wheel. In such case, the pilot pressure is maintained at a constant value and the pressure control valve serves for allowing introduction and draining of pressurized fluid in the working chamber of the hydraulic cylinder depending upon the pressure difference between the pilot pressure and the supply pressure which reflects the fluid pressure in the working chamber.
In such a suspension control system, substantially high response characteristics in adjusting the fluid pressure in the hydraulic cylinder is required so that a satisfactory response in suppression of the suspension characteristics is achieved according to the vehicle driving condition.
The proportioning solenoid is generally connected to a driver circuit to receive driver current to drive a valve position for adjusting the pilot pressure at a desired pressure. The driver circuit adjusts a supply current toward a target current which is derived on the basis of a suspension control signal input from a control unit. Frequency characteristis of the supply current and the target current are so adjusted to have a linear relationship. In such driver circuit, greater input gain is preferred for obtaining high response characteristics. On the other hand, for absorption of vibration energy, a hydraulic system in the pressure control valve is preferably provided great input frequency characteristics versus input vibration. To achieve the high input frequency characteristics in absorbing the vibration energy, an orifice is provided in a path establishing communication between the outlet of the pressure control valve and a feedback chamber. This orifice tends to serve as resistance to the fluid flow and effective for providing high response in absorption of the vibration enegy. However, on the other hand, this orifice serves as lag factor for response characteristics in the attitude change suppressive mode operation in which the pilot pressure is controlled according to the driver current.
The relationship between the frequency characteristics versus the input current and the frequency characteristics versus input vibration are shown in FIGS. 12 and 13. Lines A1 and A2 of FIG. 10 shows preferred characteristics of gain (A1) and phase (A2) of input current frequency characteristics. As will be seen herefrom, greater input gain is preferred to obtain high response. On the other hand, lines B1 (gain) and B2 (phase) show prefered characteristics of pressure variation gain and phase of input vibration frequency characteristics. When better response characteristics are obtaining in the input vibration frequency characteristics, the orifice is provided in the feedback path, gain and phase of the input current frequency characteristics varies to that illustrated by lines C1 (gain) and C2 (phase) which show undesirable characteristics. On the other hand, when input current frequency characteristics are set to obtain the characteristics of A1 and A2, the input vibration frequency characteristics becomes that illustrated by lines D1 (gain) and D2 (phase). As will be seen from lines D1 and D2, when the input current frequency characteristics is provided high gain, pressure adjustment function in the shock absoring mode operation becomes excessive, particularly in a relatively low frequency range, to give a rough ride feeling to which degrades riding comfort.
Therefore, in the conventional system, it was difficult to achive satisfactorily high response both in attitude change, suppressing mode operation and shock absorbing mode operation.